Telephonic communication between remotely located offices of a single organization is traditionally accomplished through use of public phone lines or by leasing private lines. For occasional communication traffic, public telephone lines are quite adequate. As the volume of traffic increases, it may become economically advantageous to lease private lines. While reducing the cost compared to the public lines, the leasing of private lines remains quite expensive. In an attempt to further reduce the cost, techniques have been developed for multiplexing multiple channels of communication across a reduced number of communication lines. These techniques are well known in the art, and include synchronous time-division multiplexing (TDM) as well as information packetizing.
Multiplexers performing these techniques are placed at opposite ends of a communication path, increasing the amount of communication traffic a particular line can handle, thus reducing the cost per communication event. A large organization may have several offices scattered nationally or even globally. A private communication system meeting the needs of such an organization would typically provide one or more multiplexers at each location, leased trunk lines to interconnect the locations to each other, and a local PBX system at each location to interface internal telephones to the public phone system and the leased trunk lines.
The difficulty with this approach is that multiplexers are point to point devices, in that they are intended to be connected at the two nodes (or end points) of a single communication path. Interconnection of more than two nodes requires trunk lines to interconnect every pair of nodes, and that the multiplexers at each node be capable of handling multiple trunk lines. As the complexity of this type of system increases through the addition of nodes, the number of trunk lines required for interconnection of all pairs of nodes increases exponentially, with a corresponding exponential increase in cost.
At some point, the complexity of the system carries with it a cost which is prohibitive, and in an attempt to reduce cost, some of the contemplated trunk lines are eliminated. Communications between a pair of nodes not directly connected by a trunk line are accomplished by routing through an alternate node which is connected to both nodes in the pair between which communication is to take place. The PBX at the alternate node receives the traffic from the originating node, and routes it back through its multiplexer to another trunk line and on to the destination node.
While routing in this manner reduces the number of interconnecting trunk lines required and the associated cost, other problems and limitations are introduced. For example, if the multiplexers use a digital packetizing scheme including speech compression, then routing through multiple PBX systems would likely involve tandem encoding. Tandem encoding occurs when an analog signal is packetized and compressed to digital at the source node, decompressed and restored to analog at the intermediate node's PBX, re-packetized and re-compressed to digital, and decompressed and restored again to analog at the destination PBX. Each generation of compression and decompression introduces distortion into the resulting analog signal, and more than two generations is likely to result in distortion sufficient to render the communication unintelligible. This means that the system must provide a number of trunk lines sufficient to allow every node to communicate with every other node with no more than one intermediate routing through a PBX. This requirement limits the ability of the organization to reduce cost through the elimination of trunk lines.
In addition, traffic routing requires that each node have sufficient trunk resource not only for its own communication, but also for that of the traffic it must pass on to other nodes. Determining the amount of trunk line resource necessary for any given node thus becomes a rather complex process, in that it depends upon traffic that the node will have no direct involvement with. Further complicating the problem is the fact that each node has multiple trunk lines, each corresponding to a given remote node, and that the multiplexer to trunk line connections are "nailed up", meaning that the PBX and multiplexer use the trunk line corresponding to the desired destination node. The result of this architecture is that the traffic on each trunk line depends upon the node to node routing paths, as well as the volume of traffic being handled by that node.
The previous discussion presumes the use of multiplexers with "nailed up" PBX connections and no direct connections for passing through-traffic. However, even if the multiplexers being used support networking (i.e., pass the through-traffic without PBX tandem encoding), the connections through which the through-traffic is routed are static and at best can be altered based upon the time of day or other statistical parameters. This tends to mitigate the problems associated with "nailed up" connections, but does not eliminate them. The amount of trunk line resource needed at a node still depends upon traffic the node is not involved in and upon the routing paths being used.
These problems associated with communication routing systems of the prior art are exacerbated as the complexity of the system increases. The addition of more nodes either increases the number of trunk lines required or makes routing more complicated, or both. The more complex the routing patterns, the more dependant a node is on other node's resources, and the more difficult it is to determine the amount of resource needed for any given trunk line at each node. Increased complexity also increases the administrative burden involved in managing the line resources and the equipment involved.
It is clear that there is a need for a communication system which makes optimum use of trunk line resource, while having an architecture that minimizes trunk line expense and that allows for expansion without undue complexity or expense.